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Sorption of Chromium with Struvite During Phosphorus Recovery Ashaki A. Rouff* School of Earth and Environmental Sciences, Queens College City University of New York, 65-30 Kissena Boulevard, Flushing, New York 11367, United States S Supporting Information *

ABSTRACT: Struvite (MgNH4PO4·6H2O; MAP) precipitation is a viable means of phosphorus (P) recovery from animal and human wastes. The behavior of metal contaminants such as chromium (Cr) during struvite precipitation, however, requires consideration. Here the influence of both Cr concentration and oxidation state on sorption is assessed. The Cr content of struvite precipitated in the presence of 1−100 μM Cr as Cr(III) (22.3−3030.1 mg/kg) was higher than that of solids from Cr(VI) (4.5− 5.1 mg/kg) solutions. For 1−20 μM Cr(III) solids, scanning electron microscopy (SEM) revealed etch pit formation on struvite crystal surfaces, indicative of a surface interaction. The formation of an adsorbate was confirmed by X-ray absorption fine structure spectroscopy (XAFS). At initial concentrations ≥20 μM Cr(III), XAFS confirmed the formation of a Cr(OH)3·nH2O(am) precipitate. Fourier transform infrared (FT-IR) spectroscopy revealed that both Cr(III) and Cr(VI) sorption resulted in distortion of the PO43− tetrahedra in the mineral structure. This, combined with SEM results revealed that even at low sorbed concentrations, the Cr impurity can affect the mineral surface and structure. Thus, the initial Cr concentration and oxidation state in wastes targeted for P recovery will dictate the final Cr content, the mechanism of sorption, and impact on the struvite structure.

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INTRODUCTION Phosphorus (P) is a limited natural resource, the cycling of which is accelerated by human activity due to its use as a source of fertilizer.1 With diminishing geologic P reserves and an increasing global population, P lost in the human cycle needs to be recouped.2 The recovery of P from animal and human wastes by the precipitation of the mineral struvite (MgNH4PO4·6H2O; MAP) is a viable means of addressing this problem.3−5 Struvite can be precipitated from municipal wastewater, human urine, and animal and plant wastewater.6−11 Struvite from these sources is beneficial as a slow-release fertilizer,12 and has been found to increase the yield and P content of crops compared to other fertilizers.13,14 Ultimately, the recovery of P from wastes as struvite and its use as fertilizer introduces sustainability to the human P cycle, conserving nonrenewable geologic P reserves.1 The process also lowers the volume and nutrient content of wastewater, mitigating the environmental impact of the release of high P effluents to surface and ground waters. Research has primarily focused on optimizing the conditions for struvite recovery,15 whereas the potential for contaminants © 2012 American Chemical Society

in wastewater streams to interact with precipitating struvite has received less attention.16−18 One such contaminant is chromium (Cr), which has an average concentration of ∼11 μg/L in human urine,17,19 9 mg/kg in swine waste, and up to 2111 mg/kg in sewage sludge.20 The final Cr content can depend on the degree of pretreatment. A swine wastewater was found to contain 513 μg/L total Cr and 475 μg/L after screening, but was reduced to 37 μg/L only after passing through a 0.45-μm filter.21 For a sewage sludge effluent with a total Cr concentration of 11 mg/L the separated liquid had a Cr concentration of 198 μg/L, which was reduced simultaneously with struvite precipitation.18 For urine spiked with Cr for a concentration of 98 μg/L, approximately 21% of the Cr was found to be sorbed to struvite.17 The association of Cr with struvite can be of concern for fertilizer use. This is because fertilizers are a known source of Cr contamination to soils,22 Received: Revised: Accepted: Published: 12493

June 7, 2012 October 14, 2012 October 31, 2012 October 31, 2012 dx.doi.org/10.1021/es302296m | Environ. Sci. Technol. 2012, 46, 12493−12501

Environmental Science & Technology

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Figure 1. Percentage of Cr remaining in solution from 1 to 100 μM (a) Cr(III) and (b) Cr(VI) samples as a function of time; and (c) measured Cr concentration of the recovered solids.

and continuous application can result in accumulation23 even if fertilizer concentrations are below regulatory standards. This can not only result in soil contamination, but, because various plants are known to sequester Cr,24 there is the risk of toxicity to plants25 and ultimately higher organisms. The total Cr content of struvite is only one concern. The binding mechanism and impact on the structural properties of the mineral are also factors that require consideration. Generally speaking, determination of the sorption mechanism is necessary to assess the mobility of contaminants in the presence of minerals.26,27 Sorption of Cr with struvite may proceed by several mechanisms including incorporation into the mineral structure during precipitation, adsorption to the surface of the already precipitated mineral, or by precipitation of oversaturated Cr phases. The solid reactivity and toxicity can be influenced by oxidation state, with Cr(III) relatively immobile in the presence of solids and less toxic compared to Cr(VI), which tends to remain in solution.28 Cr(III) is known to adsorb to the monohydrate dittmarite (MgNH4PO4·H2O),29 and has been doped into the structure of the Zn analog ZnNH4PO4·6H2O.30 Cr(VI) can facilitate substitution of Cr for P in mineral structures,31 and can compete with P during active uptake in plants.25 Regardless of oxidation state and sorption mechanism, even concentrations of ∼4 mg/kg Cr were found to affect the decomposition of struvite, with implications for the release of nutrients from the mineral if used as fertilizer.32 With these considerations in mind, the aim of this study is to evaluate the sorption of Cr with struvite as influenced by aqueous concentration and oxidation state. The limits of Cr sorption, the relevant sorption mechanisms, and the impact on the morphology and structure of struvite are assessed. Due to the variability in the types of wastes from which struvite can be precipitated, and the range of measured Cr concentrations, simplified model systems are used as a first step in understanding these processes. Results will elucidate the interactions of Cr with struvite, providing a better understanding of the effect of contaminants in waste streams on the struvite substrate. This in turn will advance the use of struvite fertilizers, and the sustainable use of P.

PHREEQC33 with the minteq.v4 database and a struvite solubility product of 10−13.26.34 An initial struvite saturation index of 2 (log ΩMAP = 2) was used as this is known to result in the formation of a homogeneous struvite precipitate.35 Stock solutions of MgCl2·6H2O and (NH4)2HPO4 were prepared at concentrations so that, when mixed, the desired struvite saturation would be attained. Prior to mixing, either the magnesium solution was spiked with Cr(III) as Cr(NO3)3·9H2O or the phosphate solution was spiked with Cr(VI) as Na2CrO4 for final Cr concentrations of 1−100 μM (52−5200 μg/L). This concentration range was chosen to encompass the wide range observed in wastes, while maintaining minimum detection limits for the analytical techniques to be implemented. The stock solutions were then reacted in the presence of 0.1 g L−1 struvite seed to overcome the initial energy barrier associated with mineral nucleation. Spontaneous precipitation was allowed to proceed at pH = 7.8 ± 0.2, coinciding with the pH 7−10 range in which struvite precipitation occurs, and the pH 6.5−8 range of most wastes.6,34 The suspensions were aged for up to 30 days, and aqueous samples were retrieved intermittently. Recovered aqueous samples were filtered and analyzed for total Cr content by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Perkin-Elmer Optima DV5300 instrument. All samples were prepared in duplicate. Blanks, without added Cr (0 μM), were included for each experiment. To generate solids for subsequent analysis, a separate set of 0−100 μM Cr(III) and Cr(VI) samples were prepared. After 7 days reaction the solids were recovered by filtering the entire volume of sample. The solids were allowed to air-dry. A fraction of the solid was acid digested and the final Cr concentration was determined by ICP-OES. The remaining solid was retained for further characterization. Solid Characterization. The solids generated after 7 days reaction were characterized by several techniques. X-ray diffraction (XRD) data were collected using a Phillips XPert Pro instrument with a Cu Kα radiation source over a range of 5−60° 2θ in 0.02 step sizes with an integration time of 0.5 s. Solids were carbon coated and the morphology was examined by scanning electron microscopy (SEM) using a Hitachi S2600N instrument. Fourier transform infrared spectroscopy (FT-IR) data were collected in the 600−4000 cm−1 range using a Perkin-Elmer Spectrum 100 instrument and a universal attenuated total reflectance accessory (UATR) with a ZnSe

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MATERIALS AND METHODS Sorption Experiments. Experiments were designed to assess the sorption of Cr with actively precipitating struvite. The solution chemistry was calculated using the program 12494

dx.doi.org/10.1021/es302296m | Environ. Sci. Technol. 2012, 46, 12493−12501


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Figure 2. XRD spectra for (a) MAP based on the reference file JCPDS 15-0762; and MAP recovered from 1 to 100 μM (b) Cr(III), and (c) Cr(VI) solutions.

crystal. X-ray absorption fine structure spectroscopy (XAFS) data were collected at beamline X11A, National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), Upton, NY. The beamline was tuned to the Cr K-edge (5989 eV) using a Si(111) monochromator. Data were collected in fluorescence mode using a Stearn-Heald type Lytle detector. Soller slits and a V filter were placed between the sample and the detector to reduce elastic scattering. Data were analyzed using WinXAS v.3.236 with theoretical paths calculated from crystallographic data using FEFF8.37

concentration, removal of both Cr(III), and more subtly, that of Cr(VI) is increased. The extent of Cr removal from solution is enhanced for Cr(III) relative to Cr(VI) solutions, consistent with the anticipated solid reactivity contingent upon oxidation state.28 The greater affinity of Cr(III) for struvite can be linked to the aqueous speciation. The dominant species in solution for Cr(III) and Cr(VI) are Cr(OH)2+(aq) and CrO42−(aq), respectively. The pH at which spontaneous coprecipitation occurs is pH 7.8. At this pH the surface charge of struvite is known to be negative,38 which may facilitate a preferential initial electrostatic interaction of positively charged Cr(OH)2+(aq) compared to CrO42−(aq). In addition, low Cr(VI) loadings may be a result of competition between CrO42−(aq) and the dominant P species, HPO42−(aq), in solution for similar struvite sorption sites. Solids recovered after 7 days of reaction were chosen for further analysis as struvite precipitation is complete within this time frame. Commensurate with the trend observed for aqueous samples, the solid loading of Cr on struvite increases with initial Cr concentration (Figure 1c). The Cr(VI) loading remains low over the studied concentration range at 4.3−5.1 mg/kg. However, for Cr(III) this increase is significant over several orders of magnitude, with a range of 22.3−3030.1 mg/ kg. The greatest increase is observed between the 20 μM

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RESULTS AND DISCUSSION Sorption Experiments. In the presence of precipitating struvite, Cr sorption occurs within the first few minutes, with no significant change in uptake over the 30-day reaction time (Figure 1a,b). The exception is for the 1−5 μM Cr(III) samples where there is a slight decrease in Cr concentration in solution over time (Figure 1a). In blank experiments (0 μM Cr) precipitation of struvite is complete within a 7 day period. Assuming no changes in struvite solubility due to Cr sorption, the coprecipitation of Cr is also likely complete within this time frame. Therefore, the additional sorption for 1−5 μM Cr(III) samples over the 30-day period could be due to adsorption of Cr to already precipitated struvite. With increasing initial Cr 12495



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Figure 3. SEM images for ∼1 mm crystals of (a) unreacted MAP, and (b) 1 μM, (c) 10 μM, and (d) 20 μM Cr(III). Close-ups of etch pits for (e) 10 μM and (f) 20 μM Cr(III) solids. The maximum pit size is ∼10 μm.

sample at 354.5 mg/kg and the 50 μM sample at 1997.2 mg/kg. This increase in loading with concentration >20 μM for Cr(III) samples is indicative of oversaturation and precipitation of additional Cr(III) phases. As PHREEQC predicts increased oversaturation of Cr(OH)3(am), Cr2O3 and MgCr2O4 with increasing initial Cr(III) concentration, these are potential precipitates in the Cr(III) experiments. Precipitation of additional phases, and other mechanisms of sorption, are further investigated using XRD, SEM, and FT-IR, and are confirmed by XAFS as discussed below. X-ray Diffraction. Results from XRD analysis confirm that struvite is the dominant crystalline phase in all samples (Figure 2). Compared to the standard struvite spectrum (Figure 2a), the highest intensities in the obtained spectra are observed for the (020) and (040) reflections (Figure 2b,c). This is a result of preferred orientation due to the elongation of crystals in one plane, as previously observed for electrochemically deposited struvite.39 The addition of Cr does not have a significant effect on the diffraction pattern, except for slight variations in the intensity of the reflections. For Cr(III) solids up to 20 μM, the relative intensity of the (021) reflection increases. For both the Cr(III) and Cr(VI) solids there is a more subtle change in the intensity of the (022) reflection for most samples. As the peaks in the XRD are influenced by atomic scattering, the substitution of an impurity atom can affect the peak intensity, with higher atomic numbers resulting in greater intensity.40 Thus, the change in intensity of these peaks may be the effect of a structural Cr impurity in these solids incorporated during struvite precipitation. For some of the Cr(III) solids ≥10 μM, there is also a subtle shift in the (022) and (040) reflections to lower angles. Such shifts in the XRD patterns are known to be induced by substitutions into phosphate crystal structures.41 No additional phases are detected by XRD for any of the solids. Thus, if the increased sorption at high Cr(III) concentrations is due to the precipitation of new Cr phases, these precipitates are amorphous, and therefore not sensitive to detection by XRD. Scanning Electron Microscopy. For unreacted struvite, SEM imaging reveals elongated, slightly orthorhombic ∼1 mm crystals with grooved surfaces (Figure 3a). With the addition of

Cr, the overall shape of the crystals is not significantly affected. However, pitting and roughening of the crystal surfaces becomes apparent for Cr(III) samples (Figure 3b−f). This effect occurs more frequently with increasing concentration up to 20 μM Cr(III), at which the most significant occurrence of pitting is observed with the formation of ∼10 μm pits (Figure 3d,f). At 50 μM Cr(III), the pits are smaller and their occurrence is reduced, and minimal pitting is observed for the 100 μM Cr(III) crystals (images not shown). For Cr(VI) samples, there is no notable effect on the surface features of the struvite crystals. That pitting is observed for Cr(III), and not Cr(VI), samples is indicative of greater surface interaction of Cr(III) with struvite, at comparable concentrations, which can be attributed to higher overall sorbed concentrations. The change in surface features observed for crystals generated from 1 to 20 μM Cr(III) solutions is indicative of interaction of Cr(III) with the struvite surface. As impurities are known to inhibit struvite growth,15 pit formation can be induced by adsorption and/or formation of a Cr-enriched surface layer, which in turn perturbs mineral growth at these specific locations. This phenomenon, that is the formation of etch pits due to metal adsorption to mineral surfaces, has been previously observed, for example for Hg(II) adsorption to calcite crystals.42 The predominance of these features at 20 μM indicates that this concentration may represent the upper limit of direct interaction of Cr(III) with the struvite surface. At higher concentrations the precipitation of discrete Cr(III) phases may dominate the sorption process. This likely occurs independently of the mineral surface, thus limiting pitting and surface roughening at these concentrations. Fourier Transform Infrared Spectroscopy. Solids were subject to ATR FT-IR analysis for further mineral characterization, and to determine the effect of Cr sorption on the speciation of IR-sensitive functional groups. For the unreacted solid, the FT-IR spectrum exhibits peaks characteristic of struvite43 (Figure 4a), confirming the mineralogy, and consistent with results from XRD (Figure 2). With addition of Cr, the primary effect is observed in the ν3 antisymmetric stretching vibration of the PO43‑ molecule,43 which splits into 12496



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Figure 4. ATR FT-IR spectra for (a) unreacted MAP showing vibrations associated with dominant functional groups; the ν3 PO43− band for unreacted MAP and 1 to 100 μM (c) Cr(III) and (b) Cr(VI) solids. Dotted lines represent the peaks formed due to splitting in this region.

two components with peaks at ∼1020 cm−1 and ∼970 cm−1 (Figure 4 a,b). Both the Cr(III) and Cr(VI) samples exhibit increased splitting in this band up to an initial concentration of 20 μM. At higher concentrations there is no further change in band splitting for the Cr(III) samples. For the Cr(VI) samples, splitting is reduced for solids generated from 50 to 100 μM solutions. Splitting in the ν3 PO43− band is a result of distortion in the PO43− tetrahedron,44 and is reflective of a lowering of the symmetry of the ion, and potentially that of the crystal lattice.43 This effect is known to be induced by substitution of foreign ions into phosphate mineral structures.45−47 That this splitting increases for solids up to 20 μM initial Cr(III), with no further change, suggests that this concentration is the limit of Cr(III) substitution into the struvite structure. At higher concentrations, though substitution still may occur, the oversaturation and preferential precipitation of Cr(III) phases likely dominates. As Cr(III) forms cationic species in solution, it is anticipated that substitution takes place at the Mg(II) site, as

observed for Cr(III)-doped ZnNH4PO4·6H2O.30 Substitutions can be facilitated by charge compensation mechanisms involving Cr(OH)2+(aq), Mg2+(aq), H2O(aq) and NH4+(aq) in solution.32 For the Cr(VI) solids it is notable that though overall sorbed Cr(VI) is low (≤5.1 mg/kg), there is still an effect on the symmetry of the PO43− tetrahedron. The splitting in the ν3 PO43− band is maximized at 20 μM indicating that this concentration may also be the limit of Cr(VI) substitution. Here substitution may occur at the P site, but may also be limited by competition between CrO42−(aq) and HPO42−(aq) in solution, as described earlier. At highest initial concentrations, since precipitation of Cr(VI) solids is unlikely, reduced splitting may be due to a change in the distribution between substituted and surface-bound species. This change in the ν3 PO43− band is apparent even though the final Cr(VI) concentrations are all similar, within ±0.3 ppm. This suggests that the initial Cr(VI) concentration in solution can directly influence the type of interaction with the struvite mineral. 12497



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Figure 5. EXAFS (a) k3-weighted chi spectra and (b) Fourier transforms for 5 to 100 μM Cr(III) solids.

of the 50−100 μM solids so there is also a significant contribution of Cr(OH)3·nH2O(am) to the sample. For the 5− 10 μM Cr(III) solids, Cr−Cr backscattering is reduced compared to the 20 μM solid, and the contribution of this shell decreases with decreasing concentration. Thus there is less Cr(OH)3·nH2O(am) at these concentrations, and the Cr−Cr shell may represent the formation of a precursor Cr(III) polymeric species. For these solids the coordination environment of P is 3, indicative of less surface bonds and/or a different surface configuration, compared to the 20 μM solid. A short Cr−O distance at 1.93 Å, as observed for CrPO4 is also detected. That this shell is detected separately, and not averaged with the longer Cr−O distance as observed for Cr(OH)3·nH2O(am), indicates a greater contribution of the surface-bound species to the total sorbed Cr(III) at these concentrations. These results do not preclude the formation of surface-bound Cr at 50−100 μM concentrations, however if present, these species would constitute only a small fraction of the sorbed Cr(III) relative to the Cr(OH)3·nH2O(am) precipitate. Mechanisms of Cr Sorption with Struvite. The sorption of Cr with precipitating struvite is mechanistically complex, involving adsorption and/or near surface enrichment, bulk substitution, and the precipitation of Cr-bearing phases, contingent upon reaction conditions. Cr(III) is more particle reactive compared to Cr(VI), with significant removal of the former with the solid phase. At initial aqueous concentrations of 1−20 μM, Cr(III) bonds directly to the struvite surface. This may proceed by adsorption and/or formation of a Cr-enriched surface layer. The polymerization of Cr(III) increases with concentration, and a secondary Cr(OH)3·nH2O(am) precipitate dominates at 50−100 μM. Some substitution of Cr(III) into the structure may occur at all concentrations. Initial aqueous concentrations of 1−100 μM Cr(VI) result in minimal

X-ray Absorption Fine Structure Spectroscopy. XAFS data were collected for 5−100 μM Cr(III) samples only as low concentrations of sorbed Cr for the 1 μM Cr(III) solid and the Cr(VI) reacted solids precluded analysis by this technique. The X-ray absorption near-edge (XANES) spectra confirm that sorbed Cr is in the +3 oxidation state (Figure S1). This is consistent with the initial oxidation state added in solution, therefore no redox reactions occurred during the experimental process. The extended X-ray absorption fine structure (EXAFS) chi functions and corresponding radial distribution functions for the samples are displayed in Figure 5. Results from EXAFS analysis are presented in Table 1. A Cr shell is detected for all samples, confirming the formation of a precipitate. For the highest initial Cr(III) concentrations at 50−100 μM, the bond distances and coordination numbers are consistent with those obtained for Cr(OH)3·nH2O(am), and therefore this is the dominant form of Cr(III) in these samples. This is consistent with thermodynamic calculations that do predict oversaturation of Cr(OH)3(am) in the initial solutions, however it was not possible to confirm its formation by XRD as this phase is known to be X-ray amorphous. At initial concentrations ≤20 μM Cr(III), a P shell is detected at 3.40−3.49 Å, consistent with the Cr−P distance in CrPO4. Thermodynamically, a CrPO4 solid is undersaturated in solution, and SEM images show the predominance of surface features in these samples. Thus the presence of P backscatterers is indicative of direct bonding of Cr(III) to PO43− groups at the mineral surface, as an adsorbate or a Cr-enriched surface layer, in which the local coordination environment of Cr is structurally similar to that of CrPO4. For the 20 μM solid the Cr−P distance is the average of the two distances reported for CrPO4, with 6 coordinating P atoms. That Cr in this sample is coordinated by P indicates significant Cr interaction with the struvite surface, consistent with SEM results. Also, the Cr−Cr shell is comparable to that 12498



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Table 1. EXAFS Fit Results for 5 to 100 μM Cr(III) Solids and Cr(III) Standards sample 5 μM

10 μM

20 μM

50 μM

100 μM

shell

CNa

R (Å)b

σ2 (Å2)c

O O Cr P O O Cr P O O Cr P O O Cr O O Cr O

2 2 0.5 3 4 6 2 3 6 6 4 6 6 6 4 6 6 4 6

1.93 2.08 3.02 3.49 4.03 1.93 3.03 3.49 3.93 2.00 2.97 3.40 3.94 1.98 3.04 3.89 1.98 3.04 3.91

0.001 0.002 0.002 0.010 0.008 0.002 0.008 0.001 0.004 0.005 0.010 0.020 0.010 0.005 0.017 0.010 0.004 0.017 0.011

standard Cr(OH)3·nH2O(am)e

CrPO4f

shell O Cr O O O P P

CN 6 4 6 2.5 3.1 3.2 2.9

R (Å) 1.99 3.03 3.87 1.93 2.01 3.22 3.50

in the United States, in Turkey a maximum concentration of 270 mg/kg Cr is acceptable.18 Struvite precipitated from 5−20 μM Cr(III) solutions will have a Cr content within this range, and will exceed these values only at higher initial Cr(III) concentrations. However, in addition to the total Cr concentration, the final oxidation state is critical as Cr(VI) is toxic to plants even at low concentrations, while the toxicity of Cr(III) increases with concentration.25 Also of note is that plants grown in the presence of both Cr(III) and Cr(VI), at concentrations much lower than the range reported for fertilizers, can uptake and accumulate notable amounts of Cr, affecting subsequent growth and development.25 If the Cr content of struvite is deemed acceptable for use as fertilizer, the role of sorbed Cr on the surface and structural properties of struvite should not be discounted. Even at low loadings, Cr(III) can affect growth patterns at the struvite surface, and both Cr(III) and Cr(VI) distort PO43− tetrahedra in the mineral structure. This in turn can influence the stability of the mineral, and thus the decomposition of struvite, as observed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of 1−100 μM initial Cr solids.32 Though decomposition was found to occur at higher temperature for both Cr(III) and Cr(VI) reacted solids, at the point of maximum decomposition, the weight loss was higher compared to unreacted struvite. The accelerated release of nutrients upon decomposition implies that the Cr impurity can affect the predicted properties of struvite as a slow-release fertilizer and increase the probability of nutrient leaching, even at low concentrations. In addition, sorption of both Cr(III) and Cr(VI) was found to influence the final H2O(s) and NH4+ content of the mineral.32 Thus, for struvite formation from Crbearing wastes, ensuring that the final oxidation state and Cr content of struvite does not pose a contamination risk, and thus an environmental liability to soils and plants, is only one concern. Also of importance is the effect of the impurity on the mineral structure and its behavior once applied as fertilizer. These findings have implications not only for understanding the potential for Cr contamination by struvite fertilizers, but the role of sorbed Cr in the release of P, and its subsequent cycling upon fertilizer application.

E0 (eV)d 4.2

−5.3

3.3

1.2

2.2

σ2 (Å2) 0.005 0.010 0.02 0.001 0.003 0.007 0.006

a Estimated error ±: coordination number = 20%. bEstimated error ±: radial distance = 0.01 Å. cEstimated error ±: Debye-Waller factor = 0.001 Å2. dEstimated error ±: energy shift. eFit to Cr10 standard from Tang et al.48 fFit results from Aldrich et al.49

sorption, with substitution into the bulk, combined with surface interactions as potential mechanisms. These results exemplify the variable mechanisms by which Cr may interact with precipitating struvite, contingent upon oxidation state and aqueous concentration. Environmental Implications of Cr Sorption with Struvite. Though natural wastes are more compositionally complex, this study provides an initial assessment of the Crstruvite sorption process. Studies of struvite precipitation from urine and sewage effluent containing 98 μg/L and 198 μg/L Cr, respectively, detected the removal of Cr, likely as Cr(III), with struvite.17,18 The initial concentrations in these wastes are comparable to those ≤5 μM (≤260 μg/L) in the current study. At these concentrations, though final loadings are low, Cr does have a visible effect on the struvite surface, affecting crystal growth at locations to which sorption occurs. Any surfacebound or substituted Cr will be removed with struvite, and persist when used as fertilizer. Wastes can also have higher Cr concentrations greatly exceeding those in the current study.20 Though Cr may precipitate as a separate phase at elevated concentrations, isolating the Cr precipitate from struvite may prove difficult. This means that Cr that is not directly associated with the struvite mineral may also be removed with the fertilizer product. The mean Cr content of fertilizers ranges from 33 to 485 mg/kg.50 Though not directly regulated

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. XANES spectra for the Cr(NO3)·9H2O standard used to confirm the presence of Cr(III), and the 5−100 μM Cr(III) solids. This information is available free of charge via the Internet at http://pubs.acs.org/

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Ashaki.Rouff@qc.cuny.edu; phone: 718-997-3073; fax: 718-997-3299. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support was provided to A.A.R. by a PSC−CUNY Award funded by The Professional Staff Congress and The City University of New York, the NSLS Faculty Student Research Support Program, and the Woodrow Wilson National Fellowship Foundation. Use of the NSLS, BNL, was supported by the U.S. Department of Energy, Office of Science, Office of Basic 12499



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Energy Sciences, under Contract DE-AC02-98CH10886. Thanks to Y. Tang who provided the Cr(OH)3·nH2O(am) XAFS spectrum and commented on this manuscript, K. Pandya of X11A for beamline support, N. Ma for assistance during XAFS data collection, and P. Brock for assistance with SEM.

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